Forcing I/O

[angi]

Hi guys
I got a problem about how to force on a output:
could i force a latched output ON when its un-latch control is ON also?

Thanks

 

[bernie_carlton]

In general 'forces' override whatever the logic is doing but that's as far as the final physical output state. You will have to specify an exact PLC model to get a more specific answer.

 

[angi]

the PLC I am using is COmpaclogix5332E, 1769-L32E.

Thanks

 

[Ron Beaufort]

you can force the "OUTPUT" meaning the physical device located in the field either ON or OFF ... that will NOT (I repeat, NOT) change the status of the output bit/box located in the processor's tags ...

note that the popup Tool Tip in RSLogix5000 is incorrect when it says that an output "bit" is being forced ... this error confuses quite a few people - even some with considerable experience ...

 

[angi]

I need to force turn a SOL output on, and at the same time, the SOL un-latch bit also ON, after I enable the force, it did not work (the LED on SOL never ON). How to force the SOL on at this condition?

Thanks

 

[Ron Beaufort]

the Latch and/or Unlatch INSTRUCTIONS have nothing to do with the "forcing" operation ... specifically, the force is applied "downstream" of the output bit/boxes which are controlled by the ladder logic instructions ...

big ticket question: did the LED (the one associated with the solenoid address) on the front of the output module light up? ...

did you Enable the I/O Forces? ... that's a separate step which is part of the "forcing" process ...

 

[angi]

big ticket question: did the LED (the one associated with the solenoid address) on the front of the output module light up? ...

I have no chance to check this, but the LED on solenoid will not ON.

did you Enable the I/O Forces? ... that's a separate step which is part of the "forcing" process

I did this.


Thanks

 

[bernie_carlton]

I have no chance to check this, but the LED on solenoid will not ON.

Check the LED on the module. If it is on but the solenoid isn't then start checking the voltage from the card terminals to the solenoid.

 

[angi]

the solenoid did work well when I run the program (that means the LED on solenoid could ON when the output for solenoid ON), but it will not work when I want to do some test by forcing the solenoid ON.

 

[Ron Beaufort]

there's something fishy going on here ...

if the program can drive the solenoid ON, then the force should be able to drive it ON also ...

you need to post a screen shot of how you're forcing the output ... no offense intended, but smart money says you're just not using the force feature correctly ... we'll be glad to help you out - but we've got to see more in order to help you ...

 

[Tharon]

Could be that something else is stopping the power to the Solenoid coil. Like a safety relay, emergency stop relay, or some other control relay that disconnects power before or after the output.

During normal operation, this relay would be energized, and when the output turns on, power is applied to the solenoid's coil. During your testing, even though the output is forced on, power may not exist at the output because what ever supplies the power is shut off.

Get out your trusty voltmeter.

 

[bernie_carlton]

you can force the "OUTPUT" meaning the physical device located in the field either ON or OFF ... that will NOT (I repeat, NOT) change the status of the output bit/box located in the processor's tags ...

I've been bit by that in that a contact (XIC or XIO) of an outputs does not follow the forced state. It keeps the state determined by the logic (bit box).

But this is AB's method. When I work with Automation Direct's stuff I have to pay attention as the force directly affects the 'bit box' and associated contacts follow the forced state. Also, forces all disappear after a power cycle unlike AB.

 

[Ron Beaufort]

I've been bit by that in that a contact (XIC or XIO) of an outputs does not follow the forced state. It keeps the state determined by the logic (bit box).

Bernie, my friend, you have no idea how many people that little Allen-Bradley trick has bitten over the years ...

some of my students (especially the more experienced ones in the class) become quite annoyed when I won't let them use "forces" as part of their troubleshooting exercises during the first few days of the class ...

"Well, I know how a [expletive deleted] FORCE works!" is one student's memorable way of putting it ...

the truth is that I've never had ONE student yet who hasn't been fooled in one way or another by exactly what the forces DO – and more importantly what they do NOT do ...

in fact, even the guys who wrote the RSLogix5000 software got it wrong – in those confusing little popup Tool Tips for the "output" forces ...

what it all boils down to is that the force function cannot be fully understood without first understanding the differences between:

(1) the output INSTRUCTION located in the ladder logic program ...

(2) the output BIT/BOX located in the PLC's data table ... and

(3) the output DEVICE located in the field ...

no offense intended, but based on the way the Original Poster has been phrasing his/her questions, I don't think that those foundational facts have quite been mastered ...

angi ... here's a suggestion that I honestly think will help you ... set aside about 90 minutes and watch the eleven YouTube videos available from the Sample Lessons page of my website ... be sure to watch them ALL – and in NUMBER ORDER ...

then follow the link at the bottom of the Sample Lessons page and go to the Email Quizzes page ... work through the Questions – and then the Answers - for both of the quizzes on Forcing and Toggling ...

all of the material on my site is free – and you don't have to register or anything like that ... it's all written in simple terms – for the express purpose of correcting the mistakes, misconceptions, and misunderstandings that many (most?) people have about how a PLC really functions "under the hood" ...

the truth is that forces confuse a LOT of people – and many of those people have YEARS of experience behind them ... once you understand what forces DO – and what they DON'T do, then they're no harder to handle than anything else that we deal with in PLCs ... but until you DO understand them, they can play havoc when you use them as part of your troubleshooting and debugging operations ...

I hope this helps ...

 

[angi]

how to post a screen shot?

 

[sthompson]

how to post a screen shot?

http://www.plctalk.net/movies/display-pictures.html